Definition
An informal shop expression referring to the working gas (typically a mixture of mercury vapor and an inert starter gas such as argon) sealed inside a fluorescent or gas-discharge lamp tube. When voltage is applied across the tube's electrodes, this gas ionizes and conducts current, producing ultraviolet light that excites the phosphor coating on the inner wall of the tube and causes it to glow.
Plain English
The gas trapped inside a fluorescent light tube. Electricity flows through this gas, which makes the tube light up.
Context Anchor
Seen in aircraft maintenance and systems descriptions when a component depends on gas contained in a tube to work correctly.
Derivation
Gas comes from an old scientific word for an air-like substance. Tube comes from Latin for a hollow pipe or passage. Together, the phrase points to a gas being held inside a hollow part.
Why Pilots Care
Blockages or leaks in these tubes cause incorrect airspeed or altitude readings, directly affecting flight safety and instrument accuracy.
Grounding Statement
Picture a small sealed tube holding an air-like material that can be squeezed, heated, or electrically energized inside that confined space.
Intuition Check
Do not read gas here as gasoline. In this phrase, gas means an air-like substance contained in the tube.
Example Sentence 1
When the cabin fluorescent light failed to start, the technician suspected the gas inside the tube had leaked through a cracked seal.
Example Sentence 2
A leak in the static line allowed outside air to mix with the gas inside the tube, causing the altimeter to read incorrectly.